It is generally advisable to use a base in a heat developable photosensitive material for the purpose of accelerating the development caused by heat. Such bases are generally used in the form of a base precursor (i.e., a compound capable of being decomposed by heat to release a basic component) in order to increase the stability of the photosensitive material. For practical use, it is essential that the base precursor fulfills two contradictory requirements of stability at ordinary temperatures and rapid decomposition upon heating.
Specific examples of base precursors which have so far been used include ureas (described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554), urea or combined use of urea and an ammonium salt of a weak acid (described in Japanese Patent Publication No. 1699/65), hexamethylenetetramine and semicarbazides (described in U.S. Pat. No. 3,157,503), combined use of triazine compounds and carboxylic acid (described in U.S. Pat. No. 3,493,375), dicyandiamide derivatives (described in U.S. Pat. No. 3,271,155), N-sulfonylureas (described in U.S. Pat. No. 3,420,665), amineimides (described in Research Disclosure, RD-15776 (1977)) and salts of pyrolyzable acids such as trichloroacetic acid (described in British Pat. No. 998,949).
However, image forming materials containing these base precursors have the serious disadvantage that they cannot satisfy the indispensable conditions of high stability upon storage at ordinary temperatures and rapid decomposition upon development processing. For this reason, such materials cannot provide high image density, or they produce images having a greatly reduced signal to noise ration because of the release of bases during storage.